PPM1D and medulloblastoma: The two major mechanisms in which this occurs, apart from the selection for p53 mutations, are i) deletion of chromosome 17p on which the p53 gene resides, often in a process that leads to the occurrence of isochromosome 17 in human medulloblastomas (De Smaele et al., 2004), and ii) frequent overexpression of the phosphatase Wip1 that efficiently dephosphorylates and hence deactivates wild‐type p53 (Castellino et al., 2008).